A widely held view is that the developing sites of lymphopoiesis in amniote vertebrates depend on colonization by circulating stem cells that originate in the yolk-sac blood islands. It is difficult to believe that there can be a fundamental difference between the origin of lymphocytes in the amphibian and the chick (or mouse), but my experiments show persuasively that the lymphoid cells of both the thymus and bone marrow in the anuran frog (Rana pipiens) are not ontogenically derived from mesenchymal cells of the ventral blood islands (the counterpart of yolk-sac blood islands in amniotes). In an earlier study, my colleagues and I established that thymic lymphocytes of the frog arise by direct transformation of the epithelial cells in the thymic rudiment itself (see American Zoologist 15:51-61, 1975). In the proposed study, we shall attempt to reinforce our previous finding with a new experimental approach. Chimeric frogs will be experimentally produced by cutting two embryos in half transversely and then interchanging the front halves. It is anticipated that the anterior and posterior halves of different embryos can be flawlessly united, and that such a chimeric embryo can develop into a frog of normal form. Moreover, an embryo of triploid constitution (3n equals 39), produced by pressure shock, can be combined with a diploid embryo (2n equals 26). The use of chromosomally marked embryonic halves will permit an analysis of the extent to which the histogenesis of organs in each half is influenced by cells migrating from one half to the other. In particular, we can ascertain the extent to which the posteriorly located spleen and bone marrow of the femur are colonized by cells from the anteriorly located thymus. BIBLIOGRAPHIC REFERENCE: Turpen, J.B., Volpe, E.P., and Cohen, N. On the origin of thymic lymphocytes. Amer. Zool. 15:51-61, 1975.